In the operation of gas turbine combustors, nitrogenous compounds in both the fuel used and from atmospheric nitrogen fixation during combustion lead to the presence of NOx in the combustor exhaust gas (also called the flue gas). With regulations on NOx control becoming more stringent around the world, it is important for combustion turbines to minimize NOx emissions.
One solution for reducing NOx emissions is the SCR system, which adds a reductant, typically ammonia or urea, to the exhaust gas stream before passing it through a catalytic bed that selectively adsorbs nitrogen oxides and the reducing agent. The adsorbed components undergo a chemical reaction on the catalyst surface and the reaction products are desorbed. NOx reduction using ammonia typically occurs through the following stoichiometric reactions:4NO+4NH3+O2→4N2+6H2O2NO2+4NH3+O2→3N2+6H2ONO+NO2+2NH3→2N2+3H2O
For a SCR system, the reactivity of the catalyst is dependent upon the flue gas temperature entering the catalyst reactor. The flue gas temperature at the exit of the gas turbine is typically above about 1200° F. while SCRs are generally designed to operate efficiently between about 600° F. to about 850° F. When the flue gas temperature at the exit of the gas turbine falls outside of the designed temperature range, the SCR catalytic conversion efficiency drops. Consequently, more ammonia or catalyst volume may be needed to maintain the conversion rate of the NOx, resulting in higher costs.
Two designs have been popular for cooling the exhaust gas to within the operating temperature range of the SCR. One such design cools the flue gas using a heat exchanger in the exhaust conduit, a representative example of which is described in U.S. Pat. No. 4,353,207. The heat exchanger reduces the temperature of the exhaust gas to within the reaction range by extracting heat, which is then used to produce steam for application elsewhere in the gas turbine. This type of arrangement is acceptable for combined cycle gas turbines used for base load operations, but is less applicable for simple cycle gas turbines typically used in peak load operations where space and cost are more restrictive.
Another design, applicable to both simple and combined cycle turbines, is to cool the exhaust gas by mixing it with ambient air. Using air-fans and injection openings, ambient air is blown into the exhaust gas conduit against the backpressure created by the exhaust gas flow. To reduce the temperature of the exhaust gas to within the reaction range, the quantity of air required is typically about 40-50% of the exhaust gas volume. The significant volume of coolant increases installation and operational costs while reducing turbine efficiency due to a greater pressure drop across the system. Poor mixing between the high-volume air and the exhaust gas can also cause a non-uniform temperature distribution across the catalyst bed, thus reducing overall catalytic activity. Additionally, the air flow rate may require wide-range adjustments to accommodate changing ambient conditions and exhaust gas temperatures, leading to significant operational variances.
Thus, there exists a need for a more efficient system for cooling the combustor exhaust gas to the temperature range suitable for NOx reduction in the SCRs.